DIY LCD Backlight Brightness - Page 2

Cocide · 04-07-2008, 04:20 PM

Well I was going to say I got the math wrong, but just take all the mm you see in my post and make that meters w/o converting the numbers. Basically I did the math for meters and they are tagged as mm

And for those who are curious, pilots display using the formula listed is at 54.9mcds

maybe were off by 100 fold?

nobb · 04-07-2008, 11:24 PM

If you are talking about the formula I made up off the top of my head on the first page...I think its wrong. I just realized that they measure led brightness "on axis", so total brightness is actually much more. According to wikipedia, to convert candela to to lumens (or nits) you need to divide by the radiation angle.

This is starting to get complicated. Maybe its easier to just say that 24+ leds is good.

Cocide · 04-08-2008, 08:29 AM

ya I agree, but it was a simple glimmer of hope formula and it seemed to be logical.

Well I think what I am going to do is buy at least 250 (shipping is the worst part) and basically play with them.

I will try to make at least two different ways to control the brightness and see how many are really needed (granted as screen size changes that will too, but most people seem to be using a 7" and I will be doing an 8").

When all is said and done I will post my howto. Keep in mind I need to get started on pulling my engine and painting my car first so my money will be devoted there for the next month or so. I did not think that these forms would be so active otherwise I would have posted this as I came closer to modifying my screen.

So if anyone can explain the math in a simple manor that makes it easy to plug numbers in thats great, otherwise I will just wing it and hope that those LEDs have a good amount of brightness by the time 200 are used.

It looks like I could end up with a cheap sunlight readable display after all....

pilot · 04-08-2008, 01:01 PM

Quote: Originally Posted by Cocide

Have you tried using a variable output voltage regulator? I think that using 2 pots and a photovoltaic cell it might be passable to have it automated based on the ambient light.

So am I correct to understand that with 50 LEDs it is reasonably readable in direct sunlight?

Out of curiosity, what screen are you using?

I will try an automated auto dim circuit based on the ambient light, i use LM317, which is a variable output voltage regulator.50 LEDs ( 3.5v - 3A ), it is reasonably readable in direct sunlight.It must be equivalent to 600+Nits, i think .If you add all 3M films , the final outcome is awesome!
The screen is Eby 701 7''.

Cocide · 04-08-2008, 01:25 PM

Sounds great. I do not know from memory if a variable voltage reg and a photovoltaic cell will just match up (with a few other pots to tune it) of if it will need some other mods.

I will be attempting the same thing so if you want to try it now go for it (and posting the outcome would be nice) otherwise give me a few days to do a theoretical and a few weeks for the real thing.

Cocide · 04-08-2008, 04:07 PM

So every time I said photovoltaic I meant photoresistor.

I thought I had a LM117 laying around but I didn't so building a test in kinda hard, but by looking at the data sheet and knowing that more light means less resistance (I got 20 ohms w/ a maglight and ~3 mega ohms under a blanket) I can kinda guess.

If photoresistor went on R1 and a variable resistor went on R2 to control the overall scale you would end up with an exponentially increasing brightness.

I would imagine that putting another variable resistor in parallel with the 2 would offer some control as to the maximum brightness and would make the whole thing have a more square root style curve (brightness increases more between dark and some ambient then it will from mid day to when light hits the sensor).

I would also imagine that putting a variable resistor with in parallel with the photoresistor would allow for an adjustable limit and a more square root style curve of the overall resistance of the photoresistor and variable resistor.

I would definitely put a zener diode between the regulated out and ground so that if the Vout is greater then 3.5 it trips and shorts out the system, it will save the LEDs but it might kill the voltage regulator. A better solution might be to run the whole system after a 3.6v regulator or something (if my memory is correct then a voltage regulator needs higher input then the output, but I might be wrong) that way the system can't fry your LEDs.

EDIT: There is only one place to put the photoresistor to get the brightness to increase with the amount of light, I removed the way that makes brightness increase in a dim room.

meh722 · 04-25-2008, 01:42 AM

I have a NEC 10.4 LCD that I want to convert to LED Backlit so its brighter. I also shocked myself off the inverter. It definitely got my attention.

I have some programming background with a little bit of electronics knowledge. I love the idea of using a photoresistor to control light output based on light coming into it. As a hobby I play with the Arduino board (http://arduino.cc) which is a cheap USB programmable board for $35.

I was thinking once the CCFL is convertered to LEDs to control the leds by reading the value of the photoresistor in the Arduino board so instead of controlling it with a potentiometer (although that would be good to have one for scale) it would be automated. The coding side and hardware interfacing would be easy, the CCFL -> LED Backlit is where I will have trouble.

stonestatue · 05-03-2008, 11:26 AM

This seems like a very good idea. It can be a very inexpensive solution to people that aren't afraid to crack open their LCDs and do a bit of work.

Did you say it would be necessary to replace the existing backlight with these LED, or would the LEDs supplement the backlight already in?

Also, is it possible to control the brightness level like you do with a standard car's instrument panel? This is achieved using a variable resistor. If that is possible, would a variable resistor be used, or could PWM be used for more efficiency?

Last but not least, if it is indeed necessary to replace the backlight that comes with the LCD, would it be necessary to leave the LEDs on at all times to be able to view the monitor, or could they be turned off when lighting conditions are favorable (like at night)? Road Runner has a feature that allows you to select a "day" and "night" skin, and I'm assuming brightness level has something to do with that.

Cocide · 05-08-2008, 11:34 PM

Sorry it took so long to reply, things have been really busy lately with finals and moving. Later this week I will post my build, pics, and review of that lm317 photoresistor setup. But since I have no display yet it will be a single led and if it 'works' at all for the intended purpose.

Quote:

Did you say it would be necessary to replace the existing backlight with these LED, or would the LEDs supplement the backlight already in?

That would probably be limited by space, if there was enough room to fit a row of leds on either side then I would guess that it would work well, my only concern is if the defuser will do a good job since (I'm guessing) it was not designed to have light from other places and give even light.

Quote:

Also, is it possible to control the brightness level like you do with a standard car's instrument panel? This is achieved using a variable resistor. If that is possible, would a variable resistor be used, or could PWM be used for more efficiency?

I'm sure it can be done, but it would end up being more complex.... probably. If 12v leds were used, or enough in series to take 12v then you could just hook the positive to the dimmer (assuming it is a standard car with ground as the frame/body and that the dimmer controls the positive). Since a car usually runs around 13-14v when on I would also use a 12v regulator to bring the max voltage down. If the dimmer is not attached to anything else you could also just use it for the pot in that variable regulator. The biggest problem I can see is during the day my dash lights are not on, yours might be, but unless the system is on it would not light up the display. I do like the idea that it could reduce the light output when on, but that would mean that 0v is full, 12v is just bellow full, and 10v is dimmed some... ect. So it would mean a rather complex circuit, but since you mentioned pwm I'm guessing that sounds as fun to you as it dose to me.

As far as pwm, leds can be controlled with pwm, but the pulse must be fast enough to make it seem as they are still on the whole time. I do not think it would be any more efficient, but it would... err should work just fine. The only problem I can think of is if you are drawing too much amperage for what ever you are using for pwm you would need some solid state relay or something, which probably will make it more expensive by a few bucks.

Quote:

Last but not least, if it is indeed necessary to replace the backlight that comes with the LCD, would it be necessary to leave the LEDs on at all times to be able to view the monitor, or could they be turned off when lighting conditions are favorable (like at night)? Road Runner has a feature that allows you to select a "day" and "night" skin, and I'm assuming brightness level has something to do with that.

Well without any form of backlight source, or transreflective display during the day, the only reason you can see the screen is the backlight. If you remove the ccfl tubes and turn off the leds then it would appear off unless you have some other light source.